In many applications, it is necessary to estimate the location of objects in their environment. To date, numerous location-determination systems have been proposed for this task. One such system is the global positioning system (GPS). This system includes a number of satellites that orbit the Earth. It also includes GPS receivers, monitoring stations, and differential GPS receivers on Earth.
GPS satellites transmit signals from which GPS receivers can estimate their locations on Earth. A GPS satellite signal typically includes a composition of: (1) carrier signals, (2) pseudorandom noise (PRN) codes, and (3) navigation data. GPS satellites transmit at two carrier frequencies. The first carrier frequency is approximately 1575.42 MHz, while the second is approximately 1227.60 MHz. The second carrier frequency is predominantly used for military applications.
Each satellite uses two PRN codes to modulate the first carrier signal. The first code is a coarse acquisition (C/A) code, which repeats every 1023 bits and modulates at a 1 MHz rate. The second code is a precise (P) code, which repeats on a seven-day cycle and modulates at a 10 MHz rate. Different PRN codes are assigned to different satellites in order to distinguish GPS signals transmitted by different satellites.
The navigation data is superimposed on the first carrier signal and the PRN codes. The navigation data is transmitted as a sequence of frames. This data specifies the time the satellite transmitted the current navigation sequence. The navigation data also provides information about the satellite""s clock errors, the satellite""s orbit (i.e., ephemeris data) and other system status data. A GPS satellite receives its ephemeris data from monitoring stations that monitor ephemeris errors in its altitude, position, and speed.
Based on the signals transmitted by the GPS satellites, current GPS techniques typically estimate the location of a GPS receiver by using a triangulation method. This method typically requires the acquisition and tracking of at least four satellite signals at the 1.57542 GHz frequency.
Traditional GPS acquisition techniques try to identify strong satellite signals by performing IQ correlation calculations between the GPS signal received by a GPS receiver and the C/A code of each satellite, at various code phases and Doppler-shift frequencies. For each satellite, the acquisition technique records the largest-calculated IQ value as well as the code phase and Doppler-shift frequency resulting in this value. After the IQ calculations, traditional acquisition techniques select at least four satellites that resulted in the highest-recorded IQ values for tracking at the code phases and Doppler values associated with the recorded IQ values.
After signal acquisition, a signal tracking method extracts navigation data transmitted by each selected satellite to estimate the selected-satellite""s pseudorange, which is the distance between the receiver and the selected satellite. As mentioned above, each tracked satellite""s navigation data specifies the satellite""s transmission time. Consequently, a satellite-signal""s transmission delay (i.e., the time for the signal to travel from the satellite to the receiver) can be calculated by subtracting the satellite""s transmission time from the time the receiver received the satellite""s signal. In turn, the distance between the receiver and a selected satellite (i.e., a selected satellite""s pseudorange) can be computed by multiplying the selected satellite""s transmission delay by the speed of light.
Traditional triangulation techniques compute the location of the GPS receiver based on the pseudoranges and locations of the selected satellites. These techniques can compute the location of each selected satellite from the ephemeris data. Theoretically, triangulation requires the computation of pseudoranges and locations of only three satellites. However, GPS systems often calculate the pseudorange and location of a fourth satellite because of inaccuracies in time measurement.
Some GPS systems also improve their accuracy by using a differential GPS technique. This technique requires the operation of differential GPS receivers at known locations. Unlike regular GPS receivers that use timing signals to calculate their positions, the differential GPS receivers use their known locations to calculate timing errors due to the signal transmission path. These differential GPS receivers determine what the travel time of the GPS signals should be, and compare them with what they actually are. Based on these comparisons, the differential GPS receivers generate xe2x80x9cerror correctionxe2x80x9d factors, which they relay to nearby GPS receivers. The GPS receivers then factor these errors into their calculation of the transmission delay.
Prior GPS techniques have a number of disadvantages. For instance, to perform their triangulation calculations, these techniques typically require acquisition of signals from four satellites. However, it is not always possible to acquire four satellite signals in certain locations. For example, inside structures or under foliage, the satellite signals can attenuate to levels that are not detectable by traditional signal-acquisition techniques.
Also, traditional techniques detect the code phases of the GPS satellites in a decoupled manner (i.e., they detect the code phase of each satellite individually). This approach also considers many code phase candidates that are impossible. In addition, this approach does not discount spurious peaks in correlations due to inter-satellite interference. Such interference is especially problematic when some of the satellite signals are greatly attenuated and others are not.
Therefore, there is a need for a global positioning system with improved sensitivity, which can operate in environments that cause high signal attenuation. There is further a need for a global positioning system that discounts spurious peaks in correlations due to inter-satellite interference. In addition, there is a need for a global positioning system that can perform its position-detection operation with a relatively small amount of data. More generally, there is a need for a location-determination system that addresses some or all of the above-mentioned needs.
Some embodiments of the invention provide a location-determination system that includes a number of transmitters and at least one receiver. Based on a reference signal received by the receiver, this location-determination system identifies an estimated location of the receiver within a region. In some embodiments, the system selects one or more locations within the region. For each particular selected location, the system calculates a metric value that quantifies the similarity between the received signal and the signal that the receiver could expect to receive at the particular location, in the absence or presence of interference. Based on the calculated metric value or values, the system then identifies the estimated location of the receiver.